Biogenic amine transporters are responsible for the reuptake and recycling of dopamine, norepinephrine and serotonin after their release from neurons. Plasma membrane biogenic amine transporters belong to a large family of Na+ and Cl- dependent transporters for neurotransmitters and amino acids. Inhibition of biogenic amine transporters by substances such as antidepressants or cocaine has profound behavioral effects, as does exchange mediated by the transporters between their natural substrate and amphetamine derivatives. In particular, the serotonin transporter (SERT) is sensitive to inhibition by cocaine and antidepressant drugs like imipramine and fluoxetine (Prozac), and exchanges intracellular serotonin (5- hydroxytryptamine, 5-HT) with external amphetamine and its analogs such as 3,4- methylenedioxymethamphetamine (MDMA), also known as "ecstasy". Although much progress has been made in understanding the reaction catalyzed by SERT and its coupling to Na+, Cl- and K+ ions, the fundamental mechanism remains only a hypothesis that the transporter contains a binding site with alternating access to intracellular and extracellular media. Recently, a high resolution crystal structure has become available for a prokaryotic homologue of SERT. This structure provides a model for experiments designed to understand mechanistic and structural properties of mammalian neurotransmitter transporters. This application will examine the predictions of the structural model and will use that model to investigate the mechanism of serotonin transport.